CppTools: Resolve absolute files in working copy always successfully

...since we can always provide the contents for such files.

This fixes the soft assert

    SOFT ASSERT: "document()" in file
	src/plugins/cpptools/cppsnapshotupdater.cpp, line 199

that occurs if you Follow Symbol on an include directive of the type
"ui_*.h" file that is not yet generated on disk.

Change-Id: I1e40db95a648a0570c6ccbcf5475691094097e18
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cpppreprocessor.cpp

@@ -201,8 +201,11 @@ bool CppPreprocessor::getFileContents(const QString &absoluteFilePath,
 
 bool CppPreprocessor::checkFile(const QString &absoluteFilePath) const
 {
-    if (absoluteFilePath.isEmpty() || m_included.contains(absoluteFilePath))
+    if (absoluteFilePath.isEmpty()
+            || m_included.contains(absoluteFilePath)
+            || m_workingCopy.contains(absoluteFilePath)) {
         return true;
+    }
 
     const QFileInfo fileInfo(absoluteFilePath);
     return fileInfo.isFile() && fileInfo.isReadable();